Variable transformers have been used for many years to regulate low frequency AC voltage to be fed into an inductive load such as a transformer or motor. These variable transformers suffer from many disadvantages. 0.5 to 300 KVA variable transformers are heavy. They are expensive. They are mechanical (prone to wear). They are not practical for applications where the output voltage needs to be changed quickly.
There have been other attempts at a solid-state replacement for a variable transformer, but most cause extreme distortion of the output wave. Also, most other solid state solutions are placed in series with the load therefore the current delivered to the load can never be greater than the current drawn from the source. In the current invention when operating at a low duty cycle it has the advantage of behaving as a step down transformer to deliver greater current to the load than it draws from the source.
Our inventions provide a solid-state circuit that is lightweight and inexpensive. It is able to change its output voltage quickly.
In a preferred embodiment, Insulated Gate Biploar Transistors (IGBT's) are used as the switching elements, but other types of solid state switches may be used as well, such as MOSFETs and Bipolar power transistors. The transistors we use contain an internal diode between the emitter and collector. If desired, devices without diodes may be used, but a fast recovery diode must be connected externally.